Communication systems in, or over, a geographic area typical have a plurality of communication system sites (sites) that re-use a limited number of communication resources to improve the communication system's call capacity. To ensure that re-use of communication resources does not cause interference between calls, sites allocated the same communication resources are geographically separated. By having sufficient geographic separation, the signal strength of other calls using the same communication resource is negligible in comparison, such that very little, if any, interference between the calls will result. However, the geographic separation needed to ensure an adequate interference ratio (negligible interference between calls) limits the call capacity of the communication system because not all of the available communication resources may be used at each site.
To enhance the efficiency of communication resource re-use and to improve call capacity, the sites are divided into sectors, wherein each sector contains a percentage of the available communication resources. By having the sites divided into sectors, the required geographic separation may be reduced while maintaining an adequate interference ratio. For example, U.S. Pat. No. 4,128,740, assigned to Motorola, Inc. discloses a four cell-six sector communication resource re-use pattern. As disclosed, each site is divided into six sectors and each sector contains approximately 1/24 of the available communication resources. For every four sites, the sector-communication resource pattern is repeated, and the sites are orientated to minimize geographic separation while maintaining an adequate interference ratio.
To further improve the call capacity of the communication system, where a call may be any conveyance of information via a communication resource, the communication system may utilize time division multiplexing (TDM). Typically, TDM allows a frequency, or frequency pair, to support several calls simultaneously. This is possible because only a fraction of a digital frame is needed to accurately represent an analog signal, thus several analog signals may be represented within one digital frame. For example, if the digital frame contains four representations of analog signals, a TDM communication system may accommodate four times the number of calls than that of a communication system which only allows one call per frequency, or frequency pair.
The increase in call capacity of a TDM communication system is not without cost. For example, each communication unit affiliated with a TDM communication system must have a high signal to noise ratio and a high interference ratio such that each analog signal representation is recaptured. If the analog signal representations are not recaptured, call degradation will occur. Further, in metropolitan areas, where call traffic may be very high, a TDM communication system may not have the call capacity to accommodate all of the call requests in an appropriate time frame. Therefore, a need exists for a communication system with an improved call capacity that also reduces the signal to noise ratio and the interference ratio requirements of the communication units.